The fundamental unit of crystalline silicate structures is a tetrahedral complex consisting of the Si cation in a tetrahedral coordination with four oxygens. In some structures, the tetrahedra link to form chains which result in fibrous needlelike structures. Single chains result when SiO.sub.4 tetrahedra are joined at one oxygen atom.
In other silicate structures, the tetrahedra are linked in layers or sheets as in mica minerals. Similar arrangement of the tetrahedra are found in clay minerals wherein two types of sheets may exist, one consisting of aluminum, iron or magnesium ions in a six-fold coordination with oxygens. These layer or sheet structures result from linking between three corners of each SiO.sub.4 tetrahedron to neighboring tetrahedra. Breck, Zeolite Molecular Sieves, John Wiley & Sons, A Wiley Interscience, Publication New York, London, Sydney, Toronto, p. 31 (1974) reports that these layer or sheet structures do not have three-dimensional stability and may expand if the layers are forced apart by water, other molecules or ions. Thus, these differ from silicates referred to as zeolites. By comparison, in the family of materials referred to as zeolites, the SiO.sub.4 tetrahedra are linked in three dimensions by a mutual sharing of all of the oxygen atoms; and thus the family of zeolites is characterized by a resulting three dimensional framework structure.
The difference in the crystallographic order of layered silicates compared to zeolites portend other differences, for example, differences in sorptive capacities, surface area and relative stability. Theoretically, it is possible to alter, e.g. increase, the sorptive capacities, surface area and stability. The ability to control these properties in the layered silicate is advantageous for practical reasons, related to the field of use of these products.
Pillaring or intercalation, the two words being used interchangeably herein, of a layered silicate can effect alteration, e.g. increase, of the properties of sorption, surface area and stability, compared to the precursor layered silicate substrate. The terms "pillar", "pillaring", "intercalated" and "intercalation" mean insertion of a material between the layers of the layered silicate substrate. Cf. Loeppert, Jr. et al, Clays and Clay Minerals 27(3), 201-208 (1979).
The idea of intercalation has been reduced to specific embodiments disclosed in e.g. U.S. Pat. Nos. 4,248,739 and 4,367,163. The present invention was made because those prior intercalation techniques were ineffectual to alter the properties of a novel synthetic layered silicate, designated MCM-20 having an X-ray diffraction pattern the significant characteristic lines of which are described in Table 1. High molecular weight polymeric cationic hydroxy metal complexes as taught by U.S. Pat. No. 4,248 739 were ineffectual to exchange into MCM-20. The product from the reaction of base MCM-20 with SiCl.sub.4 in acetylacetone as taught by U.S. Pat. No. 4,367,163 did not have improved adsorption. The base sample of MCM-20 treated by the present invention yielded, on the other hand, a material of high surface area and good crystallinity after high temperature calcination for a long period of time.